There are various forms of glasses. Among these, flat glasses are representative and have been used in various fields such as a window glass, an automobile window screen, a mirror, and the like. A flat glass may be manufactured by various techniques.
Representatively, steps of melting, agitating, stabilizing, molding and slow cooling are applied in the manufacturing of glasses.
Among these, the melting step is to melt (fuse) raw materials of glass to produce molten glass, and may be conducted in a melting furnace made of fire bricks. Such a melting furnace conventionally needs a burner so as to control the temperature of the molten glass during the melting step, and the use of the burner may generate much gas in the upper portion of the molten glass (glass melt). The gas brings into contact with the top surface of the molten glass, and some components of the glass being in reaction with the gas or having strong volatility evaporate from the top surface of the molten glass, thereby altering the characteristic of the glass or forming a heterogeneous glass comprising foreign materials.
If such a heterogeneous glass present in the surface of the molten glass is introduced in a molding process, a glass product to be finally obtained may be deteriorated in its quality. Therefore, it is necessary to completely eliminate the heterogeneous glass present in the surface of the molten glass.
As a conventional method for eliminating such a heterogeneous glass, a technique of forming an overflow zone in a melting furnace has been representatively used. In the overflow zone, a molten glass is made to overflow, thereby eliminating a heterogeneous glass present in the surface of the molten glass.
However, this technique has the problem that an overflow rate may vary depending on the height of the molten glass. For example, when operation conditions or glass properties are changed in processes prior to the overflow zone step, the overflow rate in the overflow zone may exceed or be less than the optimum rate. If the overflow rate exceeds the optimum rate in the overflow zone, a normal glass being molten as well as a heterogeneous glass may be excessively eliminated to increase glass manufacturing cost and time. On the contrary, if the overflow rate is less than the optimum rate, the elimination of the heterogeneous glass is insufficient and the quality and yield of a glass product to be obtained may be seriously deteriorated.